Homalonotus williamsi, Sandford, 2005

Sandford, Andrew C., 2005, Homalonotid trilobites from the Silurian and Lower Devonian of south-eastern Australia and New Zealand (Arthropoda: Trilobita: Homalonotidae), Memoirs of Museum Victoria 62 (1), pp. 1-66 : 19-21

Homalonotus williamsi sp. nov.

Other material. NMV P304512 from PL 6614.

Stratigraphic distribution. Macropleura band, Eden Park Formation (130–140 metres above base of unit), lowermost Notoparmella plentiensis Assemblage Zone , early-mid Ludlow.

Derivation of name. For George E. Williams, for his contribution to Victorian stratigraphy.

Diagnosis. Cephalon wide, length 0.6 times width. Glabella long, length 1.15 times width, sides more or less straight, tapering at about 15º, anterior margin broadly rounded, arc centred at 0.55 glabellar length. Glabellar length 0.92 times cranidial length. Glabellar lobation distinct, S1 and S2 expressed as deep notches adjacent to the axial furrows but very shallow abaxially, placed opposite 0.43 and 0.63 glabellar length respectively. Axial furrows moderately impressed on internal moulds (shallow on external moulds), preglabellar furrow very deeply impressed. Rostral plate with length of dorsal surface 0.11 times cephalic length, width (across rostral suture) 1.9 times length, triangular. Genae moderately swollen, lateral border furrow moderately impressed. Eye placed with midline of palpebral lobe opposite 0.67 glabellar length. Anterior branch of facial suture with posterior section straight and strongly convergent (at about 90º), anterior section evenly curving to the transverse. Rostral suture weakly convex. Pygidium triangular in outline, sides straight, converging posteriorly at about 100º. Axial furrows moderately impressed. Pygidial axis strongly convex, raised, sides converging at about 25º, continuous posteriorly with wide postaxial ridge. Pleural field with 7 ribs. Pleural and ring furrows moderately impressed, of subequal depth.

Discussion. Homalonotus williamsi closely resembles H. talenti from the lower Ludlow-lower Pr˘ídolí strata of the Heathcote area. Differences in cephalic morphology are subtle, but talenti can be distinguished in that the glabella is less elongate (glabellar length 1.1 times width) S1-S3 are extremely weakly impressed to indistinct, the axial furrows are much shallower, the preglabellar furrow and preglabellar field are notably shorter (exsag. and sag., glabellar length 0.95 times cranidial length), the eyes are more posteriorly placed (midline of palpebral lobe opposite 0.55 glabellar length), and the rostral process is wider posteriorly and only weakly convex downwards. The pygidium of williamsi is known only from fragments, but differs from that of talenti in having deeper axial, ring and pleural furrows.

In addition to the pygidial characters and the very deep, trench-like preglabellar furrows distinguishing the two Australian species from the northern-hemisphere species, Homalonotus talenti and H. williamsi also share straight and strongly convergent anterior branches of the facial sutures, in contrast to the convex outwards course of the sutures in H. knightii , H. dawsoni and H. rhinotropis . The Australian species and rhinotropis share a rounded anterior glabellar margin, differing from the transverse margin of knightii and dawsoni .

Environmental notes. See Dipleura garratti sp. nov.